Phase locked loops have varied and wide applications in communications equipment and other fields. Specifically, they can be used wherever it is necessary to synchronize the phase and/or frequency of two signals. In a typical communications type application, a phase locked loop (PLL) is used to synchronize a local oscillator to the frequency (or phase) of an incoming data signal. Phase locked loops also are used to tune a high frequency local oscillator itself to a separate, more stable, lower frequency local oscillator. For instance, in certain communications applications such as digital telecommunications applications, a very high frequency voltage controlled oscillator (VCO) signal may be necessary for synchronizing to an incoming radio frequency signal at, for example, 800 MHz. In order to very precisely control the frequency of the high frequency VCO, the VCO itself may be in a phase locked loop with a crystal oscillator, since crystal oscillators tend to be extremely accurate. However, crystal oscillators typically do not operate at high enough frequencies to be used directly for high radio frequency applications.
In the telecommunications field, incoming data or voice signals are FM modulated on a radio frequency (RF) carrier frequency of, for example, 800 MHz. The incoming data signal is brought down to the base band frequency, e.g., 0-4 KHz, in two steps. First it is frequency down converted into an intermediate frequency e.g., 70 MHz, and then the intermediate frequency (IF) signal is further down converted to the base band. Commonly, the 800 MHz RF signal is converted down to the IF frequency by means of heterodyning the incoming signal with a local oscillator signal that differs from the RF carrier frequency by an amount equal to the intermediate frequency. Thus, in the present example, a local oscillator operating at 870 MHz is needed. Since crystal oscillators that operate at such high frequencies typically are not available, it is common to utilize a voltage controlled oscillator for generating the 870 MHz local oscillator signal. However, since voltage controlled oscillators require highly accurate voltage control to maintain the local oscillator frequency with precision, the high frequency VCOs typically are embedded within a phase locked loop with a crystal oscillator operating at a much lower frequency.
Because the frequency bands dedicated to cellular telephone communications have rapidly become overloaded with communication traffic, both North America and Europe have recently added new radio frequency (RF) band ranges dedicated for cellular telephone communications. Particularly, in North America, where the 824-894 MHz band has been dedicated to cellular telephone communication use, the 1850-1990 MHz band has been added as a second cellular telephone communication band. In Europe, where 890-960 MHz has been dedicated to cellular telephone communications, 1710-1780 MHz has been added as a second frequency band for cellular telephone communications. Within each band there are a number of channels spaced at, for example, 200 KHz intervals so that multiple telephone calls can be supported in the same geographic cell simultaneously.
In view of these new bands for cellular telephone communications, there is a need for cellular telephones and other cellular communications equipment, including facsimile machines, pagers, wireless PCs, wireless modems, etc., that can receive and transmit signals within two separate RF carrier frequency bands, e.g., 824-894 MHz and 1850-1990 MHz.
Accordingly, there is a need for local oscillators which can operate at two very different RF frequencies. For instance, assuming a 70 MHz intermediate frequency, in order for a cellular telephone to be able to receive and transmit data in either band, it should be capable of generating a local oscillator signal in the 894-964 MHz range and a local oscillator signal in the 1920-2060 MHz range. However, while it is not difficult to produce an oscillator capable of generating signals at different frequencies within a tight band, such as 894-964 MHz or 1920-2060 MHz, it is not practical to produce a single oscillator that can produce signals over so broad a range as to cover 894-2060 MHz. Accordingly, multi-band operation requires special design considerations.
One method of providing multi-band capability is to simply provide two individual phase locked loops, one capable of generating local oscillator signals in the 1920-2060 MHz band and the other capable of generating local oscillator signals in the 894-964 MHz band. However, when switching from one band to the other in such a system, there is necessarily a start-up delay while the capacitors and other circuit components of the PLL charge up. In other words, when the telephone switches from one PLL to the other, there will be a long initial settling period before the VCO in the newly activated PLL locks to the desired frequency. Another disadvantage of this solution is that the use of two PLLs increases the circuit componentry in the telephone and thus also increases the size, weight and cost of the equipment.
Another option is to choose an intermediate frequency for the transceiver that is precisely halfway between the two possible operation bands, i.e., in our example in which the two bands are 824-894 MHz and 1850-1990 MHz, the intermediate frequency would be 1350 MHz. In this manner, only a single band PLL is necessary. However, such an IF frequency constraint, particularly at such a high frequency, can lead to significant overall architectural difficulties and disadvantages.
Accordingly, it is an object of the present invention to provide an improved multi-band phase locked loop.
It is another object of the present invention to provide a multi-band phase locked loop with optimal dynamics and close-in-phase-noise with minimal voltage controlled oscillator tuning voltage.
It is a further object of the present invention to provide a multi-band phase locked loop with maximized voltage controlled oscillator frequency slope.
It is yet a further object of the present invention to provide a multi-band phase locked loop with minimal additional circuitry for handling multiple bands.
It is yet another object of the present invention to provide a multi-band phase locked loop with minimal start-up-lock time upon switching from a first voltage controlled oscillator to another voltage controlled oscillator.